The present invention relates to an electron gun for a cathode ray tube which is used for CRT, electron microscope or an electron beam exposure device, and more particularly to an improvement in a cathode of the electron gun.
FIG. 19 is an enlarged sectional view of the vicinity of a cathode of a conventional electron gun as disclosed in JP-A-7-85807. In FIG. 19, reference numeral 1 denotes a heater. Reference numeral 2 denotes a sleeve composed of an inner sleeve 2A of a cylinder of molybdenum formed so as to surround the heater 1 and an outer sleeve 2B covering the inner sleeve 2A. The upper side (discharging side of electrons) of the inner sleeve is blocked. The upper end of the outer sleeve 2B is also blocked like the inner sleeve 2A whereas its center is opened. Reference numeral 3 denotes an electron dischargeable (i.e., discharge) region, and reference numeral 4 denotes a cathode pellet.
The cathode used in the above conventional electron gun is called an impregnated cathode. The cathode pellet 4 is formed of a porous substrate of tungsten (W) impregnated with aluminate compound of BaO, CaO and Al.sub.2 O.
The cathode pellet 4 is fixed on the upper central surface of the blocking portion of the inner sleeve 2A and exposed from the opening portion of the outer sleeve 2B. This exposed area of the cathode pellet 4 constitutes an electron dischargeable region 3. The sleeve 2 and cathode pellet 4 constitutes a cathode 5.
Above and apart from the cathode 5, a first grid 6 is provided. The first grid 6 is provided with a first grid electron passing through-hole 7. Above the first grid 6, a second grid 8 is arranged, which is provided with a second grid electron passing through-hole 9. The first grid 6 and the second grid 8 are formed of a conductive plate.
FIG. 20 shows an entire schematic configuration of a cathode ray tube used in the conventional electron gun. In FIG. 20, reference numeral 10 is a fluorescent screen opposed to the cathode 5.
As seen from FIG. 20, on the side of the fluorescent screen 10, the third grid 11, fourth grid 12 and fifth grid 13 are provided. These third, fourth and fifth grids are formed of a conductive plate and provided with an electron passing through-hole, respectively.
It should be noted that the cathode 5 and the plural grids 6, 8, 11, 12 and 13 are secured by a supporting body (not shown) so that they are in a proper alignment with one another.
Further, the first grid electron passing through-hole 7 and second grid electron passing through-hole 9 are formed of cylindrical holes having equal diameters located on the same axis, respectively. On the extending line of the axis, the cathode pellet 4 is located. The cathode pellet 4 is formed in a region around the above axis, and has a smaller area than that of the first grid electron passing though-hole 7.
An explanation will be given of the electron gun having the above configuration. To the first grid 6, a predetermined voltage, lower than that applied to the cathode 5, is applied. To the second grid 8, a predetermined voltage, higher than that applied to the cathode 5, is applied. In this way, by applying suitable voltages to the cathode 5, first grid 6 and second grid 8, electrons can be taken out to side of the fluorescent screen 10. The amount of electrons to be taken out, i.e., discharging current, can be adjusted by varying the voltage at the cathode 5 or first grid 6. Also, to the third grid 11, fourth grid 12 and fifth grid 13, predetermined voltages are applied. Thus, the electrons discharged from the surface of the cathode 5 by the field lens composed of the cathode 5 and plural grids 6, 8, 11, 12 and 13 are incident on the fluorescent screen 10 in their focused state.
As described above, the main configuration of the electron gun is provided with the cathode 5 for discharging electrons and plural grids 6 and 8 provided with the electron passing though-holes 7 and 9 for unidirectionally guiding the electrons discharged from the cathode 5.
FIG. 21 is a view for explaining the locus of the electrons discharged from the cathode 5, which illustrates the electron locus in the neighborhood of the cathode on its section. In FIG. 21, the abscissa represents a distance (mm) from the electron discharging plane of the cathode 5 toward the electron discharging side, and the ordinate represents the distance (mm) from the center axis on the electron discharging plane. Reference numeral 14 denote electron loci of the electrons discharged from the cathode 5 and reference symbol D denotes an equi-potential line. As seen from FIG. 21, the electrons discharged from the neighborhood of the cathode 5 provide crossover points in the vicinity of the fluorescent screen (right side of FIG. 21), whereas the electrons discharged from positions remote from the central axis Z provide cross-over points in the neighborhood of the electron discharging plane (left side in FIG. 21). Specifically, the force acting on electrons is in the direction normal to the equi-potential line. The field lens including the cathode 5, first grid 6 and second grid 8 is regarded as a spherical lens. Therefore, the electron beams discharged from the neighborhood of the center axis Z of the electron discharging plane cross over substantially at a single point. On the other hand, the electrons discharged from the positions apart from the center axis Z are subjected to the strong force directed to the center axis so that they provide cross-over points at positions nearer to the electron discharging plane than the electrons discharged from the neighborhood of the center axis Z do.
For this reason, reducing the diameter of an electron dischargeable region can eliminate the electron discharging from the positions remote from the center axis Z so that occurrence of "halo" due to the electron discharge therefrom can be reduced. Thus, the converging characteristic can be improved. In this way, the electron gun having the above configuration, in which the electron dischargeable region 3 has a smaller area than that of the first grid electron passing through-hole 7, can improve the convergence characteristic of electrons.
The first problem of the above conventional electron gun is to require the coincidence of the respective center axes of the electron dischargeable region 3, first grid passing through-hole 6 and second grid electron passing through-hole 8, which makes adjustment of axis alignment difficult.
The second problem of the conventional electron gun is as follows. In the case where the area of the electron dischargeable region 3 is made excessively smaller than that of the first grid electron passing through-hole 7, the electron convergence characteristic is not necessary improved in a greater degree than in the electron gun in which the area of the electron dischargeable region 3 is larger than that of the first grid electron passing through-hole 7.
The second problem will be described below in more detail. Where the electron dischargeable region is so large that it does not limit the electron discharging region, the electron discharging region in the electron discharging plane is determined mainly by the discharging current amount although it varies according to the type of the electron gun. For example, the electron gun used for CRT has an upper limit of the discharging current amount in a practical use according to the use and performance of CRT.
The upper limit in practical use will be explained. For example, the CRT for display monitor generally requires the image luminance as high as about 100 cd/cm.sup.2. In the case of a color monitor, electrons discharged from the electron discharging plane of the cathode are incident on the aperture grill provided with an electron passing slits or the shadow mask provided with electron passing through-holes according to the luminescent pattern of the luminescent screen. The electrons having passed through the electron passing slits or electron passing through-holes are incident on the fluorescent plane. Thus, the light flux substantially proportional to the incident amount of electrons is discharged from the fluorescent body and passes through the fluorescent glass which is a screen so that the light flux is discharged externally from the CRT.
For example, as regards a certain model of electron gun, the aperture rate of the aperture grill or shadow mask, light emitting efficiency of the fluorescent body, permeability of the fluorescent glass, etc., can be regarded constant. For this reason, the substantial maximum current amount which must be discharged from the cathode in order to obtain predetermined image luminance can be uniquely determined. The upper limit of performance will be explained below. For example, as matters now stands, generally, the CRT for HDTV (High Definition Television) does not have sufficient luminance. The sufficient luminance can be obtained by increasing the current amount discharged from the cathode. But, an increase in the current commonly deteriorates the convergence in an electron beam. On the other hand, because the HDTV displays video images with high resolution, the HDTV is required to converge the electrons discharged from the cathode so that the current amount cannot be simply increased in order to maintain the resolution constant. Thus, the approximate maximum current amount which can be discharged in order to obtain a predetermined image quality can be determined uniquely.
The maximum current required to obtain the maximum luminance in a practical range in a certain model of CRT using an electron gun is called a practical maximum current. It can be defined as follows. The maximum luminance in the practical range is a necessary and sufficient value as luminance of this kind of model or a substantial value that this model can spell out as performance in a catalogue. The luminance that the model can provide but leads to the image quality which is not practical because of greatly reduced focusing does not refer to the maximum luminance in the practical range. Even when the practical maximum current is taken out, in almost all cases, the area of the electron discharge region is not larger than that of the first grid electron passing through-hole 7 and is about 1/4 as large as thereof (i.e. 1/2 in diameter).
For example, assuming that the first grid electron passing through-hole 7 has a disk shape with a diameter of about 0.4 mm, even when the practical maximum current of the electron gun is taken out, the diameter of the electron discharging region in the electron discharging plane of the cathode may be about 0.2 mm. In this case, even when the electron dischargeable region is disk-shaped with a diameter of 0.3 mm which is smaller than that of the first grid electron passing through-hole 7, the effect of improving the focusing characteristic cannot be obtained. Further, even when the electron dischargeable region 3 has a diameter of 0.19 mm, the great effect of improving the focusing characteristic cannot be obtained because the amount of discharged electrons is little in the vicinity of the boundary of the electron discharging region.